A stateful video decoder takes complete chunks of the bytestream (e.g. Annex-B
H.264/HEVC stream, raw VP8/9 stream) and decodes them into raw video frames in
display order. The decoder is expected not to require any additional information
from the client to process these buffers.

Performing software parsing, processing etc. of the stream in the driver in
order to support this interface is strongly discouraged. In case such
operations are needed, use of the Stateless Video Decoder Interface (in
development) is strongly advised.

Single-planar API (see Single- and multi-planar APIs) and applicable structures may be
used interchangeably with multi-planar API, unless specified otherwise,
depending on decoder capabilities and following the general V4L2 guidelines.

i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
[0..2]: i = 0, 1, 2.

Given an OUTPUT buffer A, then A’ represents a buffer on the CAPTURE
queue containing data that resulted from processing buffer A.

the destination buffer queue; for decoders, the queue of buffers containing
decoded frames; for encoders, the queue of buffers containing an encoded
bytestream; V4L2_BUF_TYPE_VIDEO_CAPTURE or
V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE; data is captured from the hardware
into CAPTURE buffers.

client

the application communicating with the decoder or encoder implementing
this interface.

stream resolution in pixels aligned to codec and hardware requirements;
typically visible resolution rounded up to full macroblocks;
see also: visible resolution.

coded width

width for given coded resolution.

decode order

the order in which frames are decoded; may differ from display order if the
coded format includes a feature of frame reordering; for decoders,
OUTPUT buffers must be queued by the client in decode order; for
encoders CAPTURE buffers must be returned by the encoder in decode order.

destination

data resulting from the decode process; see CAPTURE.

display order

the order in which frames must be displayed; for encoders, OUTPUT
buffers must be queued by the client in display order; for decoders,
CAPTURE buffers must be returned by the decoder in display order.

DPB

Decoded Picture Buffer; an H.264/HEVC term for a buffer that stores a decoded
raw frame available for reference in further decoding steps.

EOS

end of stream.

IDR

Instantaneous Decoder Refresh; a type of a keyframe in an H.264/HEVC-encoded
stream, which clears the list of earlier reference frames (DPBs).

keyframe

an encoded frame that does not reference frames decoded earlier, i.e.
can be decoded fully on its own.

macroblock

a processing unit in image and video compression formats based on linear
block transforms (e.g. H.264, VP8, VP9); codec-specific, but for most of
popular codecs the size is 16x16 samples (pixels).

OUTPUT

the source buffer queue; for decoders, the queue of buffers containing
an encoded bytestream; for encoders, the queue of buffers containing raw
frames; V4L2_BUF_TYPE_VIDEO_OUTPUT or
V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE; the hardware is fed with data
from OUTPUT buffers.

PPS

Picture Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.

a point in the bytestream from which decoding may start/continue, without
any previous state/data present, e.g.: a keyframe (VP8/VP9) or
SPS/PPS/IDR sequence (H.264/HEVC); a resume point is required to start decode
of a new stream, or to resume decoding after a seek.

source

data fed to the decoder or encoder; see OUTPUT.

source height

height in pixels for given source resolution; relevant to encoders only.

source resolution

resolution in pixels of source frames being source to the encoder and
subject to further cropping to the bounds of visible resolution; relevant to
encoders only.

source width

width in pixels for given source resolution; relevant to encoders only.

SPS

Sequence Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.

To enumerate the set of coded formats supported by the decoder, the
client may call VIDIOC_ENUM_FMT() on OUTPUT.

The full set of supported formats will be returned, regardless of the
format set on CAPTURE.

Check the flags field of v4l2_fmtdesc for more information
about the decoder’s capabilities with respect to each coded format.
In particular whether or not the decoder has a full-fledged bytestream
parser and if the decoder supports dynamic resolution changes.

To enumerate the set of supported raw formats, the client may call
VIDIOC_ENUM_FMT() on CAPTURE.

Only the formats supported for the format currently active on OUTPUT
will be returned.

In order to enumerate raw formats supported by a given coded format,
the client must first set that coded format on OUTPUT and then
enumerate formats on CAPTURE.

Values returned by VIDIOC_ENUM_FRAMESIZES() for a coded pixel
format will include all possible coded resolutions supported by the
decoder for given coded pixel format.

Values returned by VIDIOC_ENUM_FRAMESIZES() for a raw pixel format
will include all possible frame buffer resolutions supported by the
decoder for given raw pixel format and the coded format currently set on
OUTPUT.

Supported profiles and levels for the coded format currently set on
OUTPUT, if applicable, may be queried using their respective controls
via VIDIOC_QUERYCTRL().

coded resolution of the stream; required only if it cannot be parsed
from the stream for the given coded format; otherwise the decoder will
use this resolution as a placeholder resolution that will likely change
as soon as it can parse the actual coded resolution from the stream.

sizeimage

desired size of OUTPUT buffers; the decoder may adjust it to
match hardware requirements.

other fields

follow standard semantics.

Return fields:

sizeimage

adjusted size of OUTPUT buffers.

The CAPTURE format will be updated with an appropriate frame buffer
resolution instantly based on the width and height returned by
VIDIOC_S_FMT().
However, for coded formats that include stream resolution information,
after the decoder is done parsing the information from the stream, it will
update the CAPTURE format with new values and signal a source change
event, regardless of whether they match the values set by the client or
not.

Important

Changing the OUTPUT format may change the currently set CAPTURE
format. How the new CAPTURE format is determined is up to the decoder
and the client must ensure it matches its needs afterwards.

This step only applies to coded formats that contain resolution information
in the stream. Continue queuing/dequeuing bytestream buffers to/from the
OUTPUT queue via VIDIOC_QBUF() and VIDIOC_DQBUF(). The
buffers will be processed and returned to the client in order, until
required metadata to configure the CAPTURE queue are found. This is
indicated by the decoder sending a V4L2_EVENT_SOURCE_CHANGE event with
changes set to V4L2_EVENT_SRC_CH_RESOLUTION.

It is not an error if the first buffer does not contain enough data for
this to occur. Processing of the buffers will continue as long as more
data is needed.

If data in a buffer that triggers the event is required to decode the
first frame, it will not be returned to the client, until the
initialization sequence completes and the frame is decoded.

If the client has not set the coded resolution of the stream on its own,
calling VIDIOC_G_FMT(), VIDIOC_S_FMT(),
VIDIOC_TRY_FMT() or VIDIOC_REQBUFS() on the CAPTURE
queue will not return the real values for the stream until a
V4L2_EVENT_SOURCE_CHANGE event with changes set to
V4L2_EVENT_SRC_CH_RESOLUTION is signaled.

Important

Any client query issued after the decoder queues the event will return
values applying to the just parsed stream, including queue formats,
selection rectangles and controls.

Note

A client capable of acquiring stream parameters from the bytestream on
its own may attempt to set the width and height of the OUTPUT format
to non-zero values matching the coded size of the stream, skip this step
and continue with the Capture Setup sequence. However, it must not
rely on any driver queries regarding stream parameters, such as
selection rectangles and controls, since the decoder has not parsed them
from the stream yet. If the values configured by the client do not match
those parsed by the decoder, a Dynamic Resolution Change will be
triggered to reconfigure them.

Call VIDIOC_G_FMT() on the CAPTURE queue to get format for the
destination buffers parsed/decoded from the bytestream.

Required fields:

type

a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

Return fields:

width, height

frame buffer resolution for the decoded frames.

pixelformat

pixel format for decoded frames.

num_planes (for _MPLANE type only)

number of planes for pixelformat.

sizeimage, bytesperline

as per standard semantics; matching frame buffer format.

Note

The value of pixelformat may be any pixel format supported by the
decoder for the current stream. The decoder should choose a
preferred/optimal format for the default configuration. For example, a
YUV format may be preferred over an RGB format if an additional
conversion step would be required for the latter.

the visible rectangle; it must fit within the frame buffer resolution
returned by VIDIOC_G_FMT() on CAPTURE.

The following selection targets are supported on CAPTURE:

V4L2_SEL_TGT_CROP_BOUNDS

corresponds to the coded resolution of the stream.

V4L2_SEL_TGT_CROP_DEFAULT

the rectangle covering the part of the CAPTURE buffer that
contains meaningful picture data (visible area); width and height
will be equal to the visible resolution of the stream.

V4L2_SEL_TGT_CROP

the rectangle within the coded resolution to be output to
CAPTURE; defaults to V4L2_SEL_TGT_CROP_DEFAULT; read-only on
hardware without additional compose/scaling capabilities.

V4L2_SEL_TGT_COMPOSE_BOUNDS

the maximum rectangle within a CAPTURE buffer, which the cropped
frame can be composed into; equal to V4L2_SEL_TGT_CROP if the
hardware does not support compose/scaling.

V4L2_SEL_TGT_COMPOSE_DEFAULT

equal to V4L2_SEL_TGT_CROP.

V4L2_SEL_TGT_COMPOSE

the rectangle inside a CAPTURE buffer into which the cropped
frame is written; defaults to V4L2_SEL_TGT_COMPOSE_DEFAULT;
read-only on hardware without additional compose/scaling capabilities.

V4L2_SEL_TGT_COMPOSE_PADDED

the rectangle inside a CAPTURE buffer which is overwritten by the
hardware; equal to V4L2_SEL_TGT_COMPOSE if the hardware does not
write padding pixels.

Warning

The values are guaranteed to be meaningful only after the decoder
successfully parses the stream metadata. The client must not rely on the
query before that happens.

Optional. Enumerate CAPTURE formats via VIDIOC_ENUM_FMT() on
the CAPTURE queue. Once the stream information is parsed and known, the
client may use this ioctl to discover which raw formats are supported for
given stream and select one of them via VIDIOC_S_FMT().

Important

The decoder will return only formats supported for the currently
established coded format, as per the OUTPUT format and/or stream
metadata parsed in this initialization sequence, even if more formats
may be supported by the decoder in general. In other words, the set
returned will be a subset of the initial query mentioned in the
Querying Capabilities section.

For example, a decoder may support YUV and RGB formats for resolutions
1920x1088 and lower, but only YUV for higher resolutions (due to
hardware limitations). After parsing a resolution of 1920x1088 or lower,
VIDIOC_ENUM_FMT() may return a set of YUV and RGB pixel formats,
but after parsing resolution higher than 1920x1088, the decoder will not
return RGB, unsupported for this resolution.

However, subsequent resolution change event triggered after
discovering a resolution change within the same stream may switch
the stream into a lower resolution and VIDIOC_ENUM_FMT()
would return RGB formats again in that case.

Optional. Set the CAPTURE format via VIDIOC_S_FMT() on the
CAPTURE queue. The client may choose a different format than
selected/suggested by the decoder in VIDIOC_G_FMT().

Required fields:

type

a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

pixelformat

a raw pixel format.

width, height

frame buffer resolution of the decoded stream; typically unchanged from
what was returned with VIDIOC_G_FMT(), but it may be different
if the hardware supports composition and/or scaling.

Setting the CAPTURE format will reset the compose selection rectangles
to their default values, based on the new resolution, as described in the
previous step.

Optional. Set the compose rectangle via VIDIOC_S_SELECTION() on
the CAPTURE queue if it is desired and if the decoder has compose and/or
scaling capabilities.

Required fields:

type

a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

target

set to V4L2_SEL_TGT_COMPOSE.

r.left, r.top, r.width, r.height

the rectangle inside a CAPTURE buffer into which the cropped
frame is written; defaults to V4L2_SEL_TGT_COMPOSE_DEFAULT;
read-only on hardware without additional compose/scaling capabilities.

Return fields:

r.left, r.top, r.width, r.height

the visible rectangle; it must fit within the frame buffer resolution
returned by VIDIOC_G_FMT() on CAPTURE.

Warning

The decoder may adjust the compose rectangle to the nearest
supported one to meet codec and hardware requirements. The client needs
to check the adjusted rectangle returned by VIDIOC_S_SELECTION().

If all the following conditions are met, the client may resume the decoding
instantly:

sizeimage of the new format (determined in previous steps) is less
than or equal to the size of currently allocated buffers,

the number of buffers currently allocated is greater than or equal to the
minimum number of buffers acquired in previous steps. To fulfill this
requirement, the client may use VIDIOC_CREATE_BUFS() to add new
buffers.

In that case, the remaining steps do not apply and the client may resume
the decoding by one of the following actions:

The actual number of allocated buffers may differ from the count
given. The client must check the updated value of count after the
call returns.

Note

To allocate more than the minimum number of buffers (for pipeline
depth), the client may query the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE
control to get the minimum number of buffers required, and pass the
obtained value plus the number of additional buffers needed in the
count field to VIDIOC_REQBUFS().

Alternatively, VIDIOC_CREATE_BUFS() on the CAPTURE queue can be
used to have more control over buffer allocation. For example, by
allocating buffers larger than the current CAPTURE format, future
resolution changes can be accommodated.

Required fields:

count

requested number of buffers to allocate; greater than zero.

type

a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

memory

follows standard semantics.

format

a format representing the maximum framebuffer resolution to be
accommodated by newly allocated buffers.

Return fields:

count

adjusted to the number of allocated buffers.

Warning

The actual number of allocated buffers may differ from the count
given. The client must check the updated value of count after the
call returns.

Note

To allocate buffers for a format different than parsed from the stream
metadata, the client must proceed as follows, before the metadata
parsing is initiated:

set width and height of the OUTPUT format to desired coded resolution to
let the decoder configure the CAPTURE format appropriately,

query the CAPTURE format using VIDIOC_G_FMT() and save it
until this step.

The format obtained in the query may be then used with
VIDIOC_CREATE_BUFS() in this step to allocate the buffers.

This state is reached after the Capture Setup sequence finishes successfully.
In this state, the client queues and dequeues buffers to both queues via
VIDIOC_QBUF() and VIDIOC_DQBUF(), following the standard
semantics.

The content of the source OUTPUT buffers depends on the active coded pixel
format and may be affected by codec-specific extended controls, as stated in
the documentation of each format.

Both queues operate independently, following the standard behavior of V4L2
buffer queues and memory-to-memory devices. In addition, the order of decoded
frames dequeued from the CAPTURE queue may differ from the order of queuing
coded frames to the OUTPUT queue, due to properties of the selected coded
format, e.g. frame reordering.

The client must not assume any direct relationship between CAPTURE
and OUTPUT buffers and any specific timing of buffers becoming
available to dequeue. Specifically:

a buffer queued to OUTPUT may result in no buffers being produced
on CAPTURE (e.g. if it does not contain encoded data, or if only
metadata syntax structures are present in it),

a buffer queued to OUTPUT may result in more than one buffer produced
on CAPTURE (if the encoded data contained more than one frame, or if
returning a decoded frame allowed the decoder to return a frame that
preceded it in decode, but succeeded it in the display order),

a buffer queued to OUTPUT may result in a buffer being produced on
CAPTURE later into decode process, and/or after processing further
OUTPUT buffers, or be returned out of order, e.g. if display
reordering is used,

buffers may become available on the CAPTURE queue without additional
buffers queued to OUTPUT (e.g. during drain or EOS), because of the
OUTPUT buffers queued in the past whose decoding results are only
available at later time, due to specifics of the decoding process.

Note

To allow matching decoded CAPTURE buffers with OUTPUT buffers they
originated from, the client can set the timestamp field of the
v4l2_buffer struct when queuing an OUTPUT buffer. The
CAPTURE buffer(s), which resulted from decoding that OUTPUT buffer
will have their timestamp field set to the same value when dequeued.

In addition to the straightforward case of one OUTPUT buffer producing
one CAPTURE buffer, the following cases are defined:

one OUTPUT buffer generates multiple CAPTURE buffers: the same
OUTPUT timestamp will be copied to multiple CAPTURE buffers.

multiple OUTPUT buffers generate one CAPTURE buffer: timestamp of
the OUTPUT buffer queued first will be copied.

the decoding order differs from the display order (i.e. the CAPTURE
buffers are out-of-order compared to the OUTPUT buffers): CAPTURE
timestamps will not retain the order of OUTPUT timestamps.

During the decoding, the decoder may initiate one of the special sequences, as
listed below. The sequences will result in the decoder returning all the
CAPTURE buffers that originated from all the OUTPUT buffers processed
before the sequence started. Last of the buffers will have the
V4L2_BUF_FLAG_LAST flag set. To determine the sequence to follow, the client
must check if there is any pending event and:

if a V4L2_EVENT_SOURCE_CHANGE event with changes set to
V4L2_EVENT_SRC_CH_RESOLUTION is pending, the Dynamic Resolution
Change sequence needs to be followed,

if a V4L2_EVENT_EOS event is pending, the End of Stream sequence needs
to be followed.

Some of the sequences can be intermixed with each other and need to be handled
as they happen. The exact operation is documented for each sequence.

Should a decoding error occur, it will be reported to the client with the level
of details depending on the decoder capabilities. Specifically:

the CAPTURE buffer that contains the results of the failed decode operation
will be returned with the V4L2_BUF_FLAG_ERROR flag set,

if the decoder is able to precisely report the OUTPUT buffer that triggered
the error, such buffer will be returned with the V4L2_BUF_FLAG_ERROR flag
set.

In case of a fatal failure that does not allow the decoding to continue, any
further operations on corresponding decoder file handle will return the -EIO
error code. The client may close the file handle and open a new one, or
alternatively reinitialize the instance by stopping streaming on both queues,
releasing all buffers and performing the Initialization sequence again.

Seek is controlled by the OUTPUT queue, as it is the source of coded data.
The seek does not require any specific operation on the CAPTURE queue, but
it may be affected as per normal decoder operation.

The decoder will start accepting new source bytestream buffers after the
call returns.

Start queuing buffers containing coded data after the seek to the OUTPUT
queue until a suitable resume point is found.

Note

There is no requirement to begin queuing coded data starting exactly
from a resume point (e.g. SPS or a keyframe). Any queued OUTPUT
buffers will be processed and returned to the client until a suitable
resume point is found. While looking for a resume point, the decoder
should not produce any decoded frames into CAPTURE buffers.

Some hardware is known to mishandle seeks to a non-resume point. Such an
operation may result in an unspecified number of corrupted decoded frames
being made available on the CAPTURE queue. Drivers must ensure that
no fatal decoding errors or crashes occur, and implement any necessary
handling and workarounds for hardware issues related to seek operations.

Warning

In case of the H.264/HEVC codec, the client must take care not to seek
over a change of SPS/PPS. Even though the target frame could be a
keyframe, the stale SPS/PPS inside decoder state would lead to undefined
results when decoding. Although the decoder must handle that case without
a crash or a fatal decode error, the client must not expect a sensible
decode output.

If the hardware can detect such corrupted decoded frames, then
corresponding buffers will be returned to the client with the
V4L2_BUF_FLAG_ERROR set. See the Decoding section for further
description of decode error reporting.

After a resume point is found, the decoder will start returning CAPTURE
buffers containing decoded frames.

Important

A seek may result in the Dynamic Resolution Change sequence being
initiated, due to the seek target having decoding parameters different from
the part of the stream decoded before the seek. The sequence must be handled
as per normal decoder operation.

Warning

It is not specified when the CAPTURE queue starts producing buffers
containing decoded data from the OUTPUT buffers queued after the seek,
as it operates independently from the OUTPUT queue.

The decoder may return a number of remaining CAPTURE buffers containing
decoded frames originating from the OUTPUT buffers queued before the
seek sequence is performed.

The VIDIOC_STREAMOFF operation discards any remaining queued
OUTPUT buffers, which means that not all of the OUTPUT buffers
queued before the seek sequence may have matching CAPTURE buffers
produced. For example, given the sequence of operations on the
OUTPUT queue:

QBUF(A), QBUF(B), STREAMOFF(), STREAMON(), QBUF(G), QBUF(H),

any of the following results on the CAPTURE queue is allowed:

{A’, B’, G’, H’}, {A’, G’, H’}, {G’, H’}.

To determine the CAPTURE buffer containing the first decoded frame after the
seek, the client may observe the timestamps to match the CAPTURE and OUTPUT
buffers or use V4L2_DEC_CMD_STOP and V4L2_DEC_CMD_START to drain the
decoder.

Note

To achieve instantaneous seek, the client may restart streaming on the
CAPTURE queue too to discard decoded, but not yet dequeued buffers.

Streams that include resolution metadata in the bytestream may require switching
to a different resolution during the decoding.

Note

Not all decoders can detect resolution changes. Those that do set the
V4L2_FMT_FLAG_DYN_RESOLUTION flag for the coded format when
VIDIOC_ENUM_FMT() is called.

The sequence starts when the decoder detects a coded frame with one or more of
the following parameters different from those previously established (and
reflected by corresponding queries):

coded resolution (OUTPUT width and height),

visible resolution (selection rectangles),

the minimum number of buffers needed for decoding.

Whenever that happens, the decoder must proceed as follows:

After encountering a resolution change in the stream, the decoder sends a
V4L2_EVENT_SOURCE_CHANGE event with changes set to
V4L2_EVENT_SRC_CH_RESOLUTION.

Important

Any client query issued after the decoder queues the event will return
values applying to the stream after the resolution change, including
queue formats, selection rectangles and controls.

The decoder will then process and decode all remaining buffers from before
the resolution change point.

The last buffer from before the change must be marked with the
V4L2_BUF_FLAG_LAST flag, similarly to the Drain sequence above.

Warning

The last buffer may be empty (with v4l2_bufferbytesused
= 0) and in that case it must be ignored by the client, as it does not
contain a decoded frame.

Note

Any attempt to dequeue more CAPTURE buffers beyond the buffer marked
with V4L2_BUF_FLAG_LAST will result in a -EPIPE error from
VIDIOC_DQBUF().

The client must continue the sequence as described below to continue the
decoding process.

Dequeue the source change event.

Important

A source change triggers an implicit decoder drain, similar to the
explicit Drain sequence. The decoder is stopped after it completes.
The decoding process must be resumed with either a pair of calls to
VIDIOC_STREAMOFF() and VIDIOC_STREAMON() on the
CAPTURE queue, or a call to VIDIOC_DECODER_CMD() with the
V4L2_DEC_CMD_START command.

Continue with the Capture Setup sequence.

Note

During the resolution change sequence, the OUTPUT queue must remain
streaming. Calling VIDIOC_STREAMOFF() on the OUTPUT queue would
abort the sequence and initiate a seek.

In principle, the OUTPUT queue operates separately from the CAPTURE
queue and this remains true for the duration of the entire resolution change
sequence as well.

The client should, for best performance and simplicity, keep queuing/dequeuing
buffers to/from the OUTPUT queue even while processing this sequence.

To ensure that all queued OUTPUT buffers have been processed and related
CAPTURE buffers are given to the client, the client must follow the drain
sequence described below. After the drain sequence ends, the client has
received all decoded frames for all OUTPUT buffers queued before the
sequence was started.

The sequence can be only initiated if both OUTPUT and CAPTURE
queues are streaming. For compatibility reasons, the call to
VIDIOC_DECODER_CMD() will not fail even if any of the queues is
not streaming, but at the same time it will not initiate the Drain
sequence and so the steps described below would not be applicable.

Any OUTPUT buffers queued by the client before the
VIDIOC_DECODER_CMD() was issued will be processed and decoded as
normal. The client must continue to handle both queues independently,
similarly to normal decode operation. This includes:

handling any operations triggered as a result of processing those buffers,
such as the Dynamic Resolution Change sequence, before continuing with
the drain sequence,

queuing and dequeuing CAPTURE buffers, until a buffer marked with the
V4L2_BUF_FLAG_LAST flag is dequeued,

Warning

The last buffer may be empty (with v4l2_bufferbytesused = 0) and in that case it must be ignored by the client,
as it does not contain a decoded frame.

Note

Any attempt to dequeue more CAPTURE buffers beyond the buffer
marked with V4L2_BUF_FLAG_LAST will result in a -EPIPE error from
VIDIOC_DQBUF().

dequeuing processed OUTPUT buffers, until all the buffers queued
before the V4L2_DEC_CMD_STOP command are dequeued,

dequeuing the V4L2_EVENT_EOS event, if the client subscribed to it.

Note

For backwards compatibility, the decoder will signal a V4L2_EVENT_EOS
event when the last frame has been decoded and all frames are ready to be
dequeued. It is a deprecated behavior and the client must not rely on it.
The V4L2_BUF_FLAG_LAST buffer flag should be used instead.

Once all the OUTPUT buffers queued before the V4L2_DEC_CMD_STOP call
are dequeued and the last CAPTURE buffer is dequeued, the decoder is
stopped and it will accept, but not process, any newly queued OUTPUT
buffers until the client issues any of the following operations:

V4L2_DEC_CMD_START - the decoder will not be reset and will resume
operation normally, with all the state from before the drain,

a pair of VIDIOC_STREAMOFF() and VIDIOC_STREAMON() on the
CAPTURE queue - the decoder will resume the operation normally,
however any CAPTURE buffers still in the queue will be returned to the
client,

Once the drain sequence is initiated, the client needs to drive it to
completion, as described by the steps above, unless it aborts the process by
issuing VIDIOC_STREAMOFF() on any of the OUTPUT or CAPTURE
queues. The client is not allowed to issue V4L2_DEC_CMD_START or
V4L2_DEC_CMD_STOP again while the drain sequence is in progress and they
will fail with -EBUSY error code if attempted.

If the decoder encounters an end of stream marking in the stream, the decoder
will initiate the Drain sequence, which the client must handle as described
above, skipping the initial VIDIOC_DECODER_CMD().

Setting formats and allocating buffers trigger changes in the behavior of the
decoder.

Setting the format on the OUTPUT queue may change the set of formats
supported/advertised on the CAPTURE queue. In particular, it also means
that the CAPTURE format may be reset and the client must not rely on the
previously set format being preserved.

Enumerating formats on the CAPTURE queue always returns only formats
supported for the current OUTPUT format.

Setting the format on the CAPTURE queue does not change the list of
formats available on the OUTPUT queue. An attempt to set a CAPTURE
format that is not supported for the currently selected OUTPUT format
will result in the decoder adjusting the requested CAPTURE format to a
supported one.

Enumerating formats on the OUTPUT queue always returns the full set of
supported coded formats, irrespectively of the current CAPTURE format.

While buffers are allocated on any of the OUTPUT or CAPTURE queues,
the client must not change the format on the OUTPUT queue. Drivers will
return the -EBUSY error code for any such format change attempt.

To summarize, setting formats and allocation must always start with the
OUTPUT queue and the OUTPUT queue is the master that governs the
set of supported formats for the CAPTURE queue.